Intelligent load shedding to counteract power system instability

Load shedding as the last resort to avoid a major power system breakdown has been utilized for a long time, mainly triggered by under frequency or under voltage relays and actuated by distribution system circuit breakers. Deregulation, dispersed generation, load growth and the ever increasing dependence on reliable electricity supply of modern society have forced the need for more "intelligent load shedding", as a means to preserve system integrity and provide acceptable system performance. The aim is to provide smooth load relief, in situations where the power system otherwise would go unstable. The paper covers theoretical work based on customer categories, load pattern and computer simulations, as well as field measurements in a radially fed distribution system, with a large impact of dispersed wind power generation. The measurements are performed with phasor measurement technology to capture also the dynamic properties of large wind power farms in a fairly weak system. The paper addresses indicators of imminent power system instability, advanced methods to smoothly reduce load -with and without communication, including some dynamic price based mechanisms, dynamic load response due to change in supply voltage or reactive power support, power system response due to change in load and impact of dispersed generation in power systems in transition towards an instability. The results in the paper are based both on theoretical studies and field measurements.